1. Field of the Invention
This invention relates to liquid crystal mixtures having desirable positive DC anisotropy.
2. Description of the Prior Art
Numerous compounds have already been proposed for use in liquid crystal displays. These generally correspond to the formula: EQU X -- A -- Y -- A -- Z (10)
in which A represents bivalent aromatic radicals, for example, benzene nuclei, which are connected together by a bridge member Y and have in the p or p' position an "electron repelling" radical X or an "electron attracting" radical Z. Thus, for example, from DT-OS 2, 306, 738 and 2, 443, 618, liquid crystal compounds of this structure are known in which two benzene nuclei A are joined together by way of the groups --CH.dbd.N, --C(O)O-- or --CH.dbd.CHC (O)O-- as bridge member Y, and in which in the p or p' position carries as group X an alkyl radical containing up to 8 or 10 carbon atoms and, as group Z, the nitrile group --CN.
By liquid crystals (LC) are generally understood those compounds which in the solid/liquid transition, have an anisotropic liquid phase in which the molecule is oriented nematically or smectically.
For details in this regard reference is made to the previously mentioned German patent specifications or to the literature mentioned therein.
For known twisted nematic displays it is preferred to use those liquid crystals which have positive DC anisotropy in the sense that the dielectric constant (DC) parallel to the axis of the molecule (.epsilon..sub..parallel.) is greater than the dielectric constant perpendicular to the axis of the molecule (.epsilon..sub..perp.). These dielectric constants are, for example, measured by the method described in Z. Naturforschung 29a (1974) 905, and the magnitude of the difference .DELTA..epsilon.=.epsilon..sub..parallel. -.epsilon..sub..perp. is a measure of the magnitude of the positive DC anisotropy.
Some nematic LC mixtures with positive DC anisotropy are known from DT-OS 2, 321, 632 and consist of (a) nematic compounds of the formula (11): ##STR3## in which A' represents the azoxy or carbonyloxy group and X' and Z' represent the same or different alkyl or alkoxy radicals containing 1 to 8 carbon atoms in a straight chain, and (b) compounds of the formula (12): EQU X" -- A" -- Y" (12)
in which A" represents one of the radicals ##STR4##
X" represents an n-alkyl radical containing 4 to 9 carbon atoms or a dialkylamino radical containing from 2 to 4 carbon atoms; and Y" represents the cyano, nitro, or trifluoromethyl group; and, optionally, (c) compounds of the formula (13): ##STR5## in which X' and Z' are the same as for formula (11).
The compounds of formula (11) have negative or very weakly positive (.DELTA..epsilon.&lt;2) DC anisotropy and are mixed with compounds (12) in order to obtain a mixture having overall a clearly positive (.DELTA..epsilon.&gt;2) DC anisotropy. Since the compounds (12) are not liquid crystals, their use in large proportions leads to a lowering of the clarification point of the LC mixture which precludes practical utilization. When small amounts are added, only moderately high positive DC anisotropy values (.DELTA..epsilon.) of at most about 15 can be achieved, thus necessitating operating voltages of about 3 V for displays using LC mixtures of this kind.
In DT-OS 2, 518, 725, LC mixtures having overall positive DC anisotropy are described. These consist of nematic LC compounds having negative DC anisotropy and containing additions of less than 15% of compounds of the formula (14): ##STR6## which are not liquid crystals. Z" represents a single bond, an oxygen bridge, or one of the groups --CH.dbd.N, --C(O)O--, --N.dbd.HC--, or --O(O)C--; X.sup.a and X.sup.b represent hydrogen or chlorine atoms, or nitrile, nitro or carboxy groups; and Y.sup.a and Y.sup.b are hydrogen atoms, or amino, hydroxy, alkyl, alkoxy, alkylamino or dialkylamino groups; while at least one of the substituents X.sup.a, X.sup.b, Y.sup.a and Y.sup.b is a polar group and none of these substituents is in the ortho position. Specific values for the DC anisotropy of the added compounds (14), which in DT-OS 2, 518, 725 are referred to throughout as isotropic, are not given in that specification.
The prior art in connection with nematic LC mixtures having overall positive DC anisotropy can thus be summarized by stating that nematic LC compounds which have negative or at most very slightly positive (.DELTA..epsilon..about.0.5) DC anisotropy, have been combined with isotropic compounds to form mixtures which have an overall moderately high positive DC anisotropy of less than 15, and, accordingly, need operating voltages of 3 V or higher.
For the practical operation of displays such as are used, for example, in digital watches, it is important that the operating voltage of the liquid crystals should be compatible with conventional small or very small batteries. For the reasons indicated above, known twisted nematic displays require operating voltages of at least 3 V. That is to say, batteries are required which comprise at least two cells of 1.5 V each, or else current transformers must be used in order to obtain the operating voltage from such a single-cell battery.
It would, therefore, be highly desirable to have available liquid crystal mixtures which have so high a positive DC anisotropy that they can be operated directly with single-cell batteries, i.e., with operating voltages of about 1.5 V. Since the operating voltage is proportional to the threshold voltage (U.sub.s) and the latter is dependent on the value of the DC anisotropy, the LC mixture would need to have an adequately high overall DC anisotropy of at least about 15, preferably about 25. Of course, it also is highly desirable that increases in the proportion of the added compound used in the mixture not excessively reduce the clarification point of the mixture.
On the basis of the prior art, it would have been expected that the polarization necessary to achieve a very high positive DC anisotropy would necessarily lead to use of isotropic compounds as additives to LC mixtures. In turn, such compounds would be expected to have a detrimental effect on the clarification point of the mixture. It would be further expected that with compounds of formula (10), no pronounced improvement would be possible such as would be necessary for the important aim of operating LC displays with a voltage of 1.5 V.